Abstract
Abstract. Acceleration of magnetosheath plasma resulting from the draping of the interplanetary magnetic field (IMF) around the magnetosphere can give rise to flow speeds that exceed that of the solar wind (VSW) by up to ~60%. Three case event studies out of 34 identified events are described. We then present a statistical study of draping-related accelerations in the magnetosheath. Further, we compare the results with the recent theory of Erkaev et al. (2011, 2012). We present a methodology to help distinguish draping-related accelerations from those caused by magnetic reconnection. To rule out magnetopause reconnection at low latitudes, we focus mainly on the positive Bz phase during the passage of interplanetary coronal mass ejections (ICMEs), as tabulated in Richardson and Cane (2010) for 1997–2009, and adding other events from 2010. To avoid effects of high-latitude reconnection poleward of the cusp, we also consider spacecraft observations made at low magnetic latitudes. We study the effect of upstream Alfvén Mach number (MA) and magnetic local time (MLT) on the speed ratio V/VSW. The comparison with theory is good. Namely, (i) flow speed ratios above unity occur behind the dawn–dusk terminator, (ii) those below unity occur on the dayside magnetosheath, and (iii) there is a good general agreement in the dependence of the V ratio on MA.
Highlights
Accelerated flows at the magnetopause or its boundary layers have traditionally received attention because they can provide a compelling observational confirmation of the occurrence of magnetic reconnection between the magnetosphere and the magnetosheath magnetic fields, as first postulated by Dungey (1961).Plasma acceleration can occur inside the magnetosheath close to, but outside, the magnetopause
We identify 34 examples of ion accelerations in the magnetosheath that are likely the result of interplanetary magnetic field (IMF) draping around the magnetosphere
As a primary data set for the interplanetary observations, we use the Richardson and Cane (2010) interplanetary coronal mass ejection (ICME) list covering the period 1997– 2010, examining magnetopause crossings made by the Geotail, Cluster, and THEMIS spacecraft during the northwardpointing phase of the interplanetary coronal mass ejections (ICMEs) listed there
Summary
Accelerated flows at the magnetopause or its boundary layers have traditionally received attention because they can provide a compelling observational confirmation of the occurrence of magnetic reconnection between the magnetosphere and the magnetosheath magnetic fields, as first postulated by Dungey (1961) (see, e.g., Paschmann et al, 1979; Sonnerup et al, 1981; Gosling et al, 1982). A characteristic feature of these events, which is useful in their identification, is that the flow tends to be perpendicular to the local magnetic field This is in sharp contrast to accelerations due to magnetic reconnection, where the flow has a large field-aligned component. We shall compare the observations and statistical trends with a MHD theory proposed recently (Erkaev et al, 2011, 2012) These authors used a semi-analytical approach to examine the magnetic and hydrodynamic forces due to draping for IMF Bz north. This theory predicts accelerations due to draping globally on the surface of the magnetopause for a given upstream MA that reach up to values of 60 % greater than the speed of the solar wind. This result was independently obtained by global MHD modeling (Lavraud et al, 2007), and through observation (Lavraud et al, 2007; Rosenqvist et al, 2007)
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